partition_allocator: preserve shards for replicas on original nodes #11077
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After one or several reallocations a replica may end up on one of the original nodes. Previously, we chose the shard for this replica anew in this case. This is awkward because the shard could change so the full replica id (i.e. broker_shard) changed even if the reallocation didn't change anything in the grand scheme of things. So this PR makes reallocations preserve the shard of replicas that end up on original nodes.
And of course this change makes it easier in the future to let nodes choose shards for the replicas they host themselves.
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